Pulsed gas lasers

ABSTRACT

A resonant power circuit which supplies pulsed power to a load, such as a pulsed laser, from a d.c. supply by cyclically charging a capacitor. The capacitor is discharged through the load on closure of a switch which should then open to allow the capacitor to re-charge. Occasionally the switch fails to open and the circuit of the invention then develops and applies a reverse voltage to the switch to force it to become open circuit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to pulsed gas lasers such as recombination lasersand transverse excited atmospheric (TEA) lasers. Such recombinationlasers are a class of gas lasers and derive most of the excitation oflasing action from recombination of ions following ionisation of a gasby an electrical discharge. Descriptions of recombination lasers arefound for example in: V V Zhukov, E L Latush, V S Mikhalenskin, M F Sem,Sov J Quantum Electron 7, 704 (1977); and descriptions of TEA lasers arein: F Collier, B Lacour, M Maillet, M Michon, J Appl Phys 52 (10), Oct1981; and RSRE Memo 4384, authors R C Hollins, D A Orchard, and A SSwanson, available from DRIC UK.

2. Description of Prior Art

A typical recombination laser comprises a cooled tube through which agas mixture, eg of He-Xe, or xenon, is flowed. At each end of the tubeare electrodes connected to capacitors. Adjacent the tube ends aremirrors, one a completely reflecting mirror, the other a partialreflector forming a light output window. Rapid discharging of thecapacitors causes ionisation of the laser gas in the tube to energylevels above a lasing energy level. After the electrical discharge pulseends, the excited gas recombines and emits light. For a He-Xe gas thislight has lines at 2.03, 2.65, 3.43, and 3.65 μm, ie in the so calledmid infra red wavelengths.

One type of pulsed gas laser is described in Patent Abstracts of JapanVol 15, No 258 (E-1084) 28-06-1991 and Japan-A-3083384. Improvedelectrode lifetime is provided by connecting the cathode of a diode tothe abode of the main discharge electrode, and the anode of the diode tothe cathode of the discharge electrode. The purpose of the diode is toprevent arc generation, with consequential surface pitting damage,between the discharge electrodes caused by reverse voltages.

SUMMARY OF THE INVENTION

The present invention improves the amount of laser light emitted by arecombination laser after the electrical discharge pulse has ended.

The invention may also improve the efficiency of other gas lasers byremoving current oscillations in the laser gas after the initial lasingpulse is completed and retaining electrical energy in the circuits forthe next discharge pulse.

The invention may also improve the pulse repetition frequency (prf) froma typical value of less than 10 Hz to about 20 kHz in some constructionsof gas lasers.

According to this invention the laser light output is improved bypreventing electrical current oscillations in the laser gas after theinitial electrical discharge pulse has ended, by use of rectifyingdiodes in the electrical circuit supplying the discharge pulse.

According to this invention a recombination pulsed gas laser comprises:

a laser tube containing a laser gaseous medium,

electrodes for causing an electrical discharge in the laser medium.

a highly reflecting mirror and laser output coupler adjacent either endof the laser tube to define a laser cavity,

electrical circuit means for supplying an electrical pulse to theelectrodes,

Characterised by diodes arranged in series and parallel with theelectrodes to prevent electrical current within the laser tube after theend of the initial electrical pulse.

According to an aspect of this invention the prf is increased by use ofa narrow bore quartz laser tube and sub-atmospheric gas pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only withreference to the accompanying drawings of which:

FIG. 1 is a cross sectional view of a He-Xe gas recombination laser.

FIG. 2 is a block diagram of the laser of FIG. 1 showing electricalcontrol circuitry.

FIGS. 3 to 6 are wavetraces showing laser output for different controlcircuitry.

FIG. 7 is a block diagram of a TEA laser showing electrical controlcircuitry.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As shown in FIG. 1 a recombination laser comprises a quartz tube 1surrounded by a jacket 2 with an annular space between the tube 1 andjacket 2. Inlet 3 and outlet 4 end structures support and locate thetube 1 and jacket. Cooling water inlet 5 and outlet pipes 6 are formedon the jacket 2 so that coolant may be flowed through the annular spaceand keep the tube 1 at a desired temperature.

The inlet end structure 3 carries inner flanges 7 which fix to thejacket 2, locate o-ring seals 8 on the tube 1, contain flange coolantpipes 9, 10, and support probe-like pointed electrodes 24 at the end ofthe tube 1. The inlet end structure 3 also includes an inlet chamber 11into which gas is fed via an inlet pipe 12, and outer flanges 13 whichsupport and locate a high reflectance gold coated copper mirror 14. Theinlet end structure 3 also carries two capacitors 15 which form part ofthe exciting circuit shown more clearly in FIG. 2.

The outlet end structure 4 carries inner flanges 16 which fix to thejacket 1, locate o-ring seals 17 on the tube 1, contain flange coolantpipes 18, 19, and support probe-like pointed Ta electrodes 25 at the endof the tube 1. The outlet end structure 4 also includes an outletchamber 20 from which gas is removed via an outlet pipe 21, and outerflanges 22 which support and locate a partly reflecting silicon mirror23 forming a laser output coupler.

For a gas mixture of 250:1 He:Xe a typical inner laser tube dimension is4 mm, cavity length (distance between mirror 14 and coupler 23) is 60cm, gas pressure 300 mbar. These values are examples only; the deviceoperates over a wide range of parameter values.

FIG. 2 shows the electrical control circuit for the laser of FIG. 1. ADC supply of 5 kvolts supplies power via a diode D4 and inductances 31,26 to a point A between two capacitors C1, C2 each of typically inFvalue. Between the DC supply lines is a thyratron 27. Connected inparallel with the two capacitors C1, C2 are three lines 28, 29, 30. Inone line 28 is the laser and a diode D1 in series; in the second line 29is a diode D2; and in the third line 30 is an inductance L and diode D3in series.

In operation to obtain a laser output, the circuit of FIG. 2 is suppliedwith DC power at 5 kvolts. This results in voltage oscillations alongthe supply lines, inductance, diode D4 and capacitor C1 and charges uppoint A between the capacitors to 10 kvolts. The outer sides of thecapacitors C1, C2 remain at zero potential due to conduction through theinductance L. When the capacitors C1, C2 are fully charged, thethyratron 27 is caused to short circuit. This causes capacitor C1 toinvert its voltage and a doubling of the voltage appearing across thelaser, ie to 20 kV. The capacitors C1, C2 discharge across the laserelectrodes. As a result the gas is raised to an ionised state above alasing level. The discharging pulse lasts for less than 1 μs and reducesto zero current as shown in the upper traces of FIGS. 3-6.

When the discharge pulse has finished the excited gas begins tode-excite and lase. After termination of the discharge pulse there stillexist significant amounts of electrical energy in the circuit which needto be dissipated. At this point in time the gas is still partly ionisedand is conductive. This, in the absence of diodes, allows passage ofcurrent through the gas with a consequential reduction or evenextinction of lasing activity.

Such an event is shown in FIG. 3, lower trace, where lasing action isreduced to zero by a reverse current through the gas immediately aftertermination of the discharge pulse, seen in the upper trace.

FIG. 4 shows operation of the laser with just diode D2 in the circuit ofFIG. 2, ie without D1 and D3. The amount of lasing action is improvedfrom that of FIG. 3. FIG. 5 shows the effect of using diode D1 only,with D2 and associated line, and diode D3 missing. Again an improvedlaser action is seen; the laser action lasts longer but at a lower levelthan for FIG. 4. FIG. 6 shows the effect of using diodes D1, D2, incircuit; the amplitude and duration of laser pulse is improved over thatobtained for the circuit of FIG. 3.

Suitable diodes D1, D2, D3 are silicon diodes type UF5408 in seriesparallel arrangement (eg RS Components catalogue number 264-311).

For a laser using He-Xe gas, laser output is improved. Using the designof FIG. 1, a high prf can be used with water cooling, and little or nogas flow through the laser tube; ie the laser can be operated as asealed system. This enables small, compact, lasers to be used in systemswhere gas recirculation is difficult or impossible.

In addition to the benefit of reducing current oscillations in the lasergas, the use of diodes may improve overall efficiency by retainingelectrical energy stored in the circuit in a form which can contributeto the next discharge pulse.

The invention may also be applied to recombination lasers having a muchlarger diameter laser tube and flowing gases. Improvement in laseroutputs for such a larger laser tube are similar to those illustrated inFIGS. 4 to 6.

Gases other than Xe may be used, eg strontium with helium in a mixtureof typically He:Sr of about 1000:1.

The invention may also be applied to transverse excited atmospheric(TEA) lasers. As shown in FIG. 7 a TEA laser 40 has a large diameterlaser tube 41 containing convex electrodes 42, 43 about 50 cm long, 0.5cm wide and spaced about 2.5 cm apart. Also inside the laser tube 41along both sides of the convex electrodes 42, 43 are a series of pointedelectrode pairs 44, 45, 46, 47 each pair being associated withcapacitors 48, 49. Mirrors (not shown) at each end of the tube 41 definea laser cavity. The laser tube 41 encloses a gas mixture of He:Xe at atypical pressure in the range 200 mbar to 20 bar.

Control circuitry includes a 10 to 30 kV supply connected via aresistance R1, capacitor C3 and diode D5 to the upper electrodes 42, 44,46 in the laser tube 41. The lower electrode 43 in the laser tube 41connects to an earth line. A resistance R2 connects across supply lines50, 51 into the laser tube 41. A spark gap 52, or other switch egthyratron, connects between the lines 50, 51.

In operation with the switch 52 open circuit capacitor C3 is charged upby the supply. No electrical current flows through the laser 40 becauseof the diode D5. When the switch 52 is closed the voltage on capacitorC3, 10 to 30 kV, appears across the laser electrodes 42 to 47. Thisresults in a sparking across the side electrodes 44, 45, 46, 47 withionisation of the laser gas. Additionally the side capacitors 48, 49become charged. A discharge occurs between the main laser electrodes 42,43 causing emission of laser light. The function of diode D5 is toprevent reverse electrical currents and therefore prevent currentoscillations in the laser gas. This action enhances recombination laseroutput.

The invention may also be applied to copper (Cu) gas lasers (not shown).These typically comprise an insulating ceramic laser tube containinglumps of Cu on its inner surface, and closed at its ends by windows.Electrodes at each end of the laser tube act to apply a voltage to Hegas contained within the tube. Exterior of the tube are fully and partlyreflecting mirrors forming a laser cavity and laser output coupler.

Such a Cu laser emits laser light when a very short voltage ramp pulseis applied to the electrodes. As the electrical discharge drops, thelaser ceases to emit light. Electrical oscillations can continue in thetube but do not result in further lasing. Using the circuitry of FIG. 2,these electrical oscillations are damped and their associated energyretained in the charging circuitry ready for the next discharge. The neteffect of this is to improve device efficiency by reducing the powersupply requirements, and offering a more portable Cu laser with smallerpower supplies.

We claim:
 1. A recombination pulsed gas laser comprising:a laser tubecontaining a laser gaseous medium, electrodes for causing an electricaldischarge in the laser medium, a highly reflecting mirror and laseroutput coupler adjacent either end of the laser tube to define a lasercavity, an electrical circuit supplying an electrical pulse to theelectrodes with at least one diode connected in series with saidelectrodes preventing electrical current within the laser tube after theend of the initial electrical pulse.
 2. The laser of claim 1 wherein thelasing gas is Xe.
 3. The laser of claim 1 wherein the lasing gas is Sr.4. The laser of claim 1 wherein the gas is a mixture of He and Xe. 5.The laser of claim 1 wherein the laser tube is a narrow bore tube.
 6. Arecombination pulsed gas laser comprising:a laser tube containing alaser gaseous medium, electrodes for causing an electrical discharge inthe laser medium, a highly reflecting mirror and laser output coupleradjacent either end of the laser tube to define a laser cavity, anelectrical circuit supplying an electrical pulse to the electrodescomprising at least one diode connected in parallel with said electrodespreventing electrical current within the laser tube after the end of theinitial electrical pulse.
 7. The laser of claim 6 wherein the lasing gasis Xe.
 8. The laser of claim 6 wherein the lasing gas is Sr.
 9. Thelaser of claim 6 wherein the lasing gas is a mixture of He and Xe. 10.The laser of claim 6 wherein the laser tube is a narrow bore tube.
 11. Arecombination pulsed gas laser comprising:a laser tube containing alaser gaseous medium, electrodes for causing an electrical discharge inthe laser medium, a highly reflecting mirror and laser output coupleradjacent either end of the laser tube to define a laser cavity, anelectrical circuit supplying an electrical pulse to the electrodescomprising at least one diode and inductor in series and said diode andinductor connected in parallel with said electrodes preventingelectrical current within the laser tube after the end of the initialelectrical pulse.
 12. The laser of claim 11 wherein the lasing gas isXe.
 13. The laser of claim 11 wherein the lasing gas is Sr.
 14. Thelaser of claim 11 wherein the lasing gas is a mixture of He and Xe. 15.The laser of claim 11 wherein the laser tube is a narrow bore tube. 16.A recombination pulsed gas laser in which a laser output is obtained byrecombination of ions in a laser gaseous medium following ionisation ofa gas by an electrical discharge pulse, said laser comprising:a lasertube having ends and containing a laser gaseous medium, electrodes incontact with the laser gaseous medium and providing ionisation of thelaser gaseous medium when supplied with an initial electrical dischargepulse, a highly reflecting mirror and a laser output coupler, eachadjacent an end of the laser tube, thereby defining a laser cavity,electrical circuit supplying an initial electrical discharge pulse tothe electrodes, at least one diode arranged outside the laser cavity andconnected electrically in parallel with the electrodes, and a diodeelectrically connected in series between the electrical circuit and oneof the electrodes, whereby electrical current oscillations in the lasergaseous medium are prevented after the initial electrical dischargepulse has ended.
 17. The laser of claim 16 wherein the lasing gas is Xe.18. The laser of claim 16 wherein the lasing gas is Sr.
 19. The laser ofclaim 16 wherein the lasing gas is a mixture of He and Xe.
 20. The laserof claim 16 wherein the laser tube is a narrow bore tube.